(1) Field of the Invention
The invention relates to the general area of field emission devices, more particularly to the question of how to stabilize the cathode currents.
(2) Description of the Prior Art
Cold cathode electron emission devices are based on the phenomenon of high field emission wherein electrons can be emitted into a vacuum from a room temperature source if the local electric field at the surface in question is high enough. The creation of such high local electric fields does not necessarily require the application of very high voltage, provided the emitting surface has a sufficiently small radius of curvature.
The advent of semiconductor integrated circuit technology made possible the development and mass production of arrays of cold cathode emitters of this type. In most cases, cold cathode field emission displays comprise an array of very small emitters, usually of conical shape, each of which is connected to a source of negative voltage via a cathode line. Another set of conductive lines (called gate lines) is located a short distance above the cathode lines at 90.degree. to them. Where these two sets of lines intersect a large number of conical emitters, or microtips, are located on the cathode lines. The gate lines are connected to a source of voltage that is positive relative to the cathode lines.
The electrons that are emitted by the cold cathodes accelerate past openings in the gate lines and strike a layer of conductive phosphor that is located some distance above the gate lines. One or more microtips thus serves as a subpixel for the total display. The number of subpixels that will be combined to constitute a single pixel depends on the resolution of the display and on the operating current that is to be used.
FIG. 1(a) is a schematic diagram of a single field emission device of the above-described setup. Microtip emitter 1 is electrically connected to cathode line 2. Gate line 3, running orthogonal to cathode line 2, is separated from line 2 by insulating layer 4 and is at the height of the tip, or apex, of emitter 1. An opening in line 3 is positioned so that emitter 1 is centrally located beneath it. A plan view of the basic components that comprise the full display is given in FIG. 2. The display panel 21 occupies essentially all of the upper surface of substrate 20. Gate lines 22 are driven by scan driver 24 (which determines when each line is powered) while cathode lines 23 are driven by data driver 25 (which determines the power available to a given line at a given time).
In general, even though the local electric field in the immediate vicinity of a microtip is in excess of several million volts/cm., the externally applied voltage is under a 100 volts. However, even a relatively low voltage of this order can obviously lead to catastrophic consequences, if short circuited. Consequently, a resistor is usually placed between either the cathode lines or the control gate lines and the power supply, as ballast to limit the current in the event of a short circuit occurring somewhere within the display. This is illustrated in FIG. 1(b) where resistor 5 has been inserted between cathode line 2 and the power supply.
Ballast resistors can also sometimes be used to alleviate a different problem. When the cathode-gate voltage is applied for the first time, or after the display has been idle for a while, it has been found that, for a fixed applied voltage, the cathode current is initially relatively low but rises for some time until it levels off at its operational, or activated, value T.sub.a. This is illustrated in FIG. 3 where curve 31 shows gate emission (in arbitrary units) vs. time in seconds for a typical example of a group of field emitting microtips. T.sub.a typically depends on the vacuum level and on the emitter surface conditions (several minutes for a vacuum of the order of 10.sup.-7 torr). By using a relatively large ballast resistor (typically of the order of several hundred megohms) the cathode-gate circuit can be made to behave as a constant current circuit. Thus, as the tip-to-gate resistance drops, a larger proportion of the applied voltage is dropped across the ballast resistor and the cathode current remains substantially unchanged.
The above described phenomenon of field emitter activation has been described by J. D. Levine et al. in a paper entitled "Field emission from microtip arrays using resistor stabilization" in J. Vac. Sci. Tech. B vol. 13 no. 2 March 1995 pp. 474-477. They attribute the initial high resistance of the field emission device to the presence of adsorbed gas at the surface of the microtip. With time, said gas slowly desorbs and the emission current gradually rises. This hypothesis was confirmed by the fact that they found that for a 10.times. decrease in base pressure there was a 3.times. increase in the initial cathode current. Sometimes, the adsorption of electronegative species, after long idle time in poor vacuum, will be more serious and will lead to longer T.sub.a. In a manufacturing environment the cost and difficulty of achieving and subsequently maintaining a vacuum less than about 10.sup.-7 torr is high and an alternative solution needs to be found. A solution may be to use a large ballast resistor, as already discussed above. However, this solution to the problem suffers from the major disadvantage that it increases the loading of the data and scan drivers. It needs more power to operate the FED device and has a longer response time.
Several patents have been issued relative to the use of ballast resistors as a means for stabilizing the initial emission current. An example of this is that of Lee et al. (U.S. Pat. No. 5,357,172 October 1994). Casper et al. (U.S. Pat. No. 5,210,472 May 1993) use a pair of series connected Field Effect Transistors to provide regulating resistance in series with each row and column of the display, but the principle is the same.